Evidence for a Cr metastable phase as a tracer in DLI-MOCVD chromium hard coatings usable in high temperature environment

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Evidence for a Cr metastable phase as a tracer in

DLI-MOCVD chromium hard coatings usable in high

temperature environment

Alexandre Michau, Francis Maury, Frederic Schuster, Raphael Boichot, Michel

Pons

To cite this version:

Alexandre Michau, Francis Maury, Frederic Schuster, Raphael Boichot, Michel Pons. Evidence for a

Cr metastable phase as a tracer in DLI-MOCVD chromium hard coatings usable in high temperature

environment. Applied Surface Science, Elsevier, 2017, 422, pp.198-206. �10.1016/j.apsusc.2017.05.253�.

�hal-01666265�

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Evidence

for

a

Cr

metastable

phase

as

a

tracer

in

DLI-MOCVD

chromium

hard

coatings

usable

in

high

temperature

environment

Alexandre

Michau

a

_,

_Francis

_Maury

a,∗

_,

_Frederic

_Schuster

b

_,

_Raphael

_Boichot

c

_,

Michel

Pons

c

a_CIRIMAT,_{CNRS/INPT/UPS,}₄_allée_E._Monso,₃₁₀₃₀_Toulouse_cedex_4,_France b_CEA_Saclay,_DFP/DPg,₉₁₁₉₁_Gif_Sur_Yvette,_France

c_University_Grenoble_Alpes,_SIMAP,_CNRS,₃₈₀₀₀_Grenoble,_France

Keywords: Chromiumcoatings MOCVD Crmetastablephase Structuraltransformation Tracer Hardcoatings

a

b

s

t

r

a

c

t

Crdepositsarewidelyusedasprotectivecoatingsbutmultifunctionalperformancesarerequiredinharsh environmentsmotivatingresearchonnewprocesses.MOCVDofCrmetalcoatingswascarriedoutby directliquidinjection(DLI)ofauniquesolutioncontainingbis(ethylbenzene)chromiumasmetalsource andthiophenolasinhibitorofcarbideformation.Alowamount(<6%)ofthemetastabled-Crphasewas foundembeddedinthestablea-Crphase.Theformationofthismetastablephaseoriginatesfromboth thelowdepositiontemperature(<723K)andtheuseofthiophenol.ItwasnotreportedunderotherCVD conditions.Densecoatingsweredepositedbyimplementingamultilayergrowthmode.Suchcoatings exhibitahighnanohardnessofabout17GPa.Thed-Crmetastablephaseundergoesanirreversible struc-turaltransformationtobcc-Crabove723K.Themechanicalpropertiesofcoatingsarenotaffectedbythe structuraltransformationbecauseofthesimilarityoftheircrystallographicstructures(bothcubic),their densityveryclose(avolumecontractionofonly0.4%duringthetransformation)anditslowcontent.This metastablephaseisasignatureoftheDLI-MOCVDprocessanditcanbeusedasatracerforCrcoatings operatinginhightemperatureenvironmentwithoutlossofthebasicproperties.

1. Introduction

Inordertogrowprotectivemetallicchromiumcoatings,wet routesgenerallyusealotoftoxicchemicalsincludingchromicacid solutionsbasedonhexavalentchromium[1–3]whichisnow for-biddenbyREACHregulation.Severalalternativesareinvestigated usingtrivalentchromium[4,5]buttheyshouldalsoresultina reg-ulationban.

Attheopposite,itispossibletodepositchromiumcoatingsby PVDprocesses ascathodicmagnetronsputtering [6,7],cathodic arcevaporation[8],hollowcathodedischarge[9]andhighpower impulsemagnetronsputtering(HIPIMS)[10]withoutusing danger-ouschemicals.Themaindrawbacksofthesephysicalmethodsare theircomplexityforthedepositiononlargepieces,therelatively lowgrowthratesand finallythefactthattheyareline-of-sight deposition techniques, which excludes conformal and uniform coatingsonthreedimensionalobjects.

∗ Correspondingauthorat:CIRIMAT,ENSIACET,4,alléeEmileMonso,BP44362, 31030,Toulousecedex4,France.

E-mailaddress:francis.maury@ensiacet.fr(F.Maury).

CVDprocessesarewellsuitedforconformalgrowthoncomplex shapes.Theirdrawbackcomesfromthehighworking tempera-turesusingtraditionalhalidesprecursors.Forinstance,Crmetal coatingsaredepositedbypackcementationabove1300K[11–13]. However,byusingmetalorganiccompounds(MOCVD),the depo-sitiontemperatureissignificantlydecreased,forinstancebelow 773KforCr-basedcoatingsdependingontheprecursor[14,15], andevenat523Kusingreactivetetra-alkylchromiumcomplexes

[16].Asaresult,thinfilmdepositiononawidevarietyofsubstrate materialsispossiblewithoutstructuralanddimensionalchanges. InspiteofaC-richvaporphaseoriginatingfromtheorganicligands oftheprecursor,metallicCrthinfilmscanbedepositedbyMOCVD usinginhibitorsofcarbonincorporationandcarbideformationas chlorinated[14,17–20]andsulfur-containingderivatives[21].

Anevenmoresurprisingchallengewastofindthatwhenthe emergingtechnologyofdirectliquidinjection(DLI)isimplemented for supplying a cold-wall reactor operating under atmospheric pressure with high vapor flow rates, the carbon incorporation inhibitorswerestilleffectivewhilelargeamountsofhydrocarbon solvent were injected with theprecursor [22–24]. Indeed DLI-MOCVDisanalternativeprocesswheretheprecursorisdilutedor dissolvedinaliquidorganicsolvent.Thissolutionistheninjected

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asapulsedsprayinaflashvaporizationchamberand the reac-tivegasphaseistransportedbyacarriergastotheCVDreactor. Thistechniquegenerateshighlystableandcontrolledvaporflow ratesoftheprecursorleadingtorelativelyhighgrowthrateatlow temperature(<773K).Anappropriatechoiceofprecursorprovides highyields[15].Thus,DLI-MOCVDprocessforCrdepositionhas beensuccessfulusingC6Cl6[22–24]andC6H5SH[24]asinhibitors

andbis(arene)chromiumasmetalsource.

Themicrostructuralcharacteristicsandthereforetheproperties ofchromiumcoatingsdependonboththeprocessesusedandthe depositionconditions.Therearetwodifferentcrystallinephases ofchromiuminstandardconditions.Thefirstonecorrespondsto thestablebcccubicstructurewithIm-3m(229)spacegroupand 2.88Åcellparameter(PDF00-006-0694),notedbcc-Cr(ora-Cr). ThesecondoneisaprimitivecubicmetastablephasewithPm-3n (223)spacegroupand4.59Åcellparameter(PDF00-019-0323)

[25,26],notedd-Cr.

Excepthigh-temperatureCVDprocessesthatproduce thermo-chemical diffusion coatings,mostof theabove cited processes, includingDLI-MOCVD,lead totheformationofa-Cr thin films, althoughthegrowthconditionsarefarfromthermodynamic equi-librium. Theprocesses leadingtothegrowthof themetastable phase are rare. Evaporation and condensation of chromium in argonatlowpressures[25–27]ledtotheformationofcrystalline metastabled-Cr.Thesameauthorsreportedthestructural trans-formationofthemetastablephasetothestableoneabove723K.

TheaimofthispaperistostudyCrcoatingsdepositedat temper-aturesbelow723Kinahot-wallDLI-MOCVDreactor.Weshowthey havetheparticularityofcontainingsmallamountsofmetastable d-Crphaseembeddedina-Crpolycrystallinematrix.Furthermore, theseCrcoatingsaresupersaturatedwithcarbonwhichgivesthem unusualmetallurgicalproperties.Theresultinghighnanohardness makesthemgoodcandidatesashardprotectivecoatings.Thiswork alsodemonstratesthatforapplicationsinharshenvironments,if theoperatingtemperatureexceeds 723K, themetastable phase irreversiblytransformsintothestablebccphase,actingasatracer withoutlossofthebasicpropertiesofthecoating.

2. Experimental 2.1. Depositionprocess

DepositionswerecarriedoutbyDLI-MOCVDin ahorizontal, hot-wall,pyrextubularreactor(300mmlongand24mminternal diameter)withanisothermalzonearound150mm.Si(100)and stainlesssteel(304L)plateswereusedassubstratesandplacedona planarhorizontalsubstrate-holderintheisothermalzone.Thetotal pressurewaskeptconstantat6.7kPaandthegrowthtemperature wasfixedateither673(400◦_C)_or₇₂₃_K₍₄₅₀◦_C).

Commercial BEBC, bis(ethylbenzene)chromium (from Strem Chemicals,CAS12212-68-9,infactamixtureof[(C2H5)xC6H6-x]2Cr

where x=0–4)wasusedaschromiumprecursor. Itis aviscous liquidthatwasmixedwithanhydroustoluene(99.8%from Sigma-Aldrich,CAS108-88-3)at3×10−1molL−1(4gofBEBCin50mL oftoluene).Asulfur-containinginhibitorofcarbonincorporation, thiophenol,C6H5SH(fromSigma-Aldrich,purity>99%),was

intro-ducedintothissolutionwithamolerationthiol/nBEBC=2%togrow

preferentiallyCrmetalcoatingsinsteadofchromiumcarbide coat-ings.

The liquid solution containing both the precursor and the inhibitorwasinjectedinaflashvaporizationchamber(473K)at 0.9mLmin−1 _by_{appropriately} _tuning_injection _parameters

(fre-quencyandopeningtime).Nitrogenwasusedascarriergaswith a500sccmflowrateandwasheatedat453Kbeforeenteringthe flashvaporizationchambertopreventcondensation.

Table1

GrowthconditionsofCrmetalcoatingsbyDLI-MOCVDinahorizontalhot-wall reactorusinganinjectionofasinglesolutionofBEBCandthiophenolintoluene,as chromiumsourceandinhibitorofcarbideformation,respectively.

Coatingtype Monolayer Multilayers (9layers) Depositiontemperature(K) 673;723 673;723 Totalpressure(kPa) 6.7 6.7 CarriergasN2(sccm) 500 500

Solvent Toluene Toluene BEBCconcentration(mol/L) 3.5×10−1 _3.5_×₁₀−1

Thiophenolconcentration(mol/L) 7.0×10−3_;_3.5_×₁₀−2 _7.0_×₁₀−3

Injectionfeedrate(mL/min) 0.9 0.4 Injectionfrequency(Hz) 10 3 Openingtimeofinjector(ms) 0.5 0.5 Growthduration(min) 60 140

Inafirstmode,coatingsweredepositedasmonolithiclayers, namelymonolayers,bykeepingconstantalltheconditionsduring thedepositionrun.Howeverthefirstanalysesrevealeda colum-narmorphologywithahighporositybetweenthecolumnswhich is not suitable for protective coatings. In order toincrease the compactness,inaseconddepositionmode,thecoatingswere struc-turedinmultilayersbystoppingtheprecursorinjectionfor5min every15minwithoutsignificanttemperaturechangeorventing. Thetemporaryshutdownofthegrowthcreatedcleaninterfaces. Atthebeginningofeachinjectionperiodanewnucleationstep andcrystalgrowthoccurredandwasstoppedbeforethe forma-tionofthecolumns,i.e.after15min,whichlimitsthethicknessof individuallayersandhindersthedevelopmentofcolumns.Thus, densemultilayercoatingswith9layerswereproduced. Character-izationswillbeshownforbothtypesofcoatings.Table1detailsthe experimentaldepositionconditions.

2.2. Coatingcharacterization

The morphology of the Cr coatings were characterized by scanning electron microscopy (SEM; Leo-435VP) and their microstructurewasstudiedbyElectronBackScatteredDiffraction (NordlysNanoEBSDDetectorwiththeAZtecHKLsoftwareinstalled ona SEM-FEGJEOLJSM-7100TTLS LV)and TEM(JEOLJEM2100 equippedwitha200kVFEG).Theircrystallinestructureand ther-malstabilitywereanalyzedbyXRDandin-situXRDvstemperature under Ar atmosphere, respectively (Bruker D8-2 diffractometer equippedwithagraphitemonochromator;Bragg-Brentano con-figuration;CuKaradiation).

Thefilmcompositionwasanalyzedbyelectronprobe micro-analysis(EPMA;CamecaSXFive,15kVand20nA)andthechemical environmentofeachelementofthecoatingwasinvestigatedusing anXPSspectrometer(ThermoScientificK-Alpha),equippedwith a monochromaticAlX-raysourceanda low energyAr+ _gun₍₁

keV) forsurfacecleaning anddepthprofileanalysis.Rutherford BackscatteringSpectrometry(RBS;1.5MeVH+_beam;_detection_at

160◦₎_was_used_to_estimate_the_density_of_coatings.

3. Resultsanddiscussion

3.1. Depositionofthebcc-Crstablephase

ManyCVDreactorconfigurationsand sizes,aswellas depo-sitionconditionshavebeenpreviouslytestedtodepositmetallic Cr [14,17–24]. These studies on MOCVD and DLI-MOCVD pro-cesses are summarized in Table 2. In the case of DLI-MOCVD process,thepresenceofalargeamountofhydrocarbonsolvent vaporisworthnoting.Experimentalsetsinvolving(i)additionof areactivegas(H2)totheinertcarriergas(N2),(ii)hot-walland

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Table2

ExperimentalconditionsusedforCrdepositionbyMOCVDandDLI-MOCVD.Bis(benzene)chromium(BBC)andbis(ethylbenzene)chromium(BEBC)wereusedasprecursors andeitherC6Cl6orC6H5SHwereusedasinhibitorsofcarbideformation.

Process Reactora _Carbon_inhibitor _BBC _BEBC

Atmb _P_(kPa) _T_(K) _Ref. _Atmb _P_(kPa) _T_(K) _Ref.

MOCVDc _hw _C 2Cl6;C6Cl6 – – – – nr nr 573–748 [17] MOCVDc _hw _(C 6H5CH2)2S;(C6H5)2S – – – – vacuum 1.3×10−3 _573–723 _[21] MOCVD cw C6Cl6 H2 0.8 673 [14] – – – – MOCVD hw C6Cl6 H2 0.4–2.0 573–673 [14,18,19] H2 0.8 623 [18]

DLI-MOCVD cw C6Cl6 Tol;Cyclo 6.7;101 753–793 [22,23] – – – –

DLI-MOCVD hw C6Cl6 Tol 6.7 748 [24] – – – –

DLI-MOCVD hw C6H5SH Tol 6.7 748 [24] – – – –

DLI-MOCVD hw C6H5SH Tol 6.7 723 Thiswork Tol 6.7 673;723 Thiswork a_cw_means_cold-wall_vertical_reactor_and_hw_means_hot-wall_horizontal_tubular_reactor.

b_Atm_means_atmosphere;_Tol_and_Cyclo_mean_toluene_and_cyclohexane_used_as_solvents_to_inject_the_precursor_solution.

c _The_precursor_used,_namely_Barkhos_was_a_mixture_of_{bis(arene)chromium}_slightly_different_from_the_BEBC_(nr_means_not_reported).

orbis(ethylbenzene)chromium(BEBC)asbis(arene)chromium(0) precursor,(iv)differentcarbideinhibitors,(v)totalpressurerange fromatmospherictolowpressure(0.4kPa),and(vi)growth tem-peraturerangefrom573to793Kwerestudied.Themainreason toexploretheeffectsofalltheseconditionscomesfromthekey roleplayedbysurfacechemistryforthecompetitivedepositionof metallicCrandcarbides.Obviouslythissurfacechemistrycanbe influencedbythegasphasechemistrywhichcanbedependenton experimentalconditionsandreactorconfigurations.

So far, MOCVD and DLI-MOCVD processes only led to the growthofsingle-phasepolycrystallinebcc-Crcoatings.Thisisthe stablebody-centered cubicstructure of themetal(spacegroup Im-3m).This common phase was obtainedby adding a chlori-nated[14,17,22,24]orasulfur-containing[21,24]inhibitortothe bis(arene)chromiumprecursor. The present work complements thesepreviousstudiesandisdistinguishedonseveralpoints:(i) theDLI-MOCVDprocesswasused,(ii)withBEBCandC6H5SHin

auniquesolutioncontainingboththeprecursorandtheinhibitor, (iii)H2wasnotaddedinthereactivegasphase,(iv)alargerCVD

reactorwasused,and(v)operatingtemperatureswerelowerthan 723K,asspecifiedinTable2.

Amonolayercoatingdepositedaccordingtomode1at723Kby DLI-MOCVDonsiliconsubstrateexhibitstypicallyacolumnarand porouschromiumstructure.Itisgenerallycomposedofthestable bcccrystallinephase.Noevidenceofcarbideformationwasfound byXRD.TheSEMmicrographofFig.1ashowsa5.7mmthickcoating

consistingofdisjointcolumnsperpendiculartothesurfaceofabout 1mmwidegrownfromadenser1mmthicksub-layerwherethe columnsarenotyetformed.ThecorrespondingXRDpatterninthe 2urange35–80◦_exhibits_the_two_main_peaks_of_the_stable_bcc-Cr_at

44.4◦₍₁₁₀₎_and_64.6◦_(200)._The_Ka2_contribution_can_also_be_seen

asashoulderofthe(200)Crpeak,aswellastheKbcontributionfor themostintense(110)Crpeak(Fig.1b).Thisstablebcc-Crphase exhibitsapreferentialorientationofthegrowthalongthe[110] directionwithatexturecoefficientof1.8whichisconsistentwith acolumnarmorphology.

Asexpected,thecoatingsdepositedaccordingtothemode2 have a multilayer structure with a greater compactness and a smoothersurfacemorphologycomparedwithmonolayercoatings.

Fig.2showsacross-sectionviewofacleavedsampleonSisubstrate grownat673K.Itconsistsof9layerswithclearinterfacesandthe totalthicknessis3.4mm.Theaveragegrainsizeislowerandless dispersedthanformode1;typicallyoftheorderofmagnitudeof thethicknessoftheindividuallayers(≤380nm).

Theinterfacesareclearbetweeneachlayerbecausethestopping oftheinjectionlastedlongenough(5min)tomakethetransient effects negligible and to suppress any nutrient species for the growthofthecoating.Indeed,theevacuationofthegaseous reac-tantsisrapidunderatotalpressureof6.7kPaandmaintainingthe carriergasflowrate(500sccm)duringthestoppingoftheprecursor injectionensuresanefficientpurgeofthereactor.

Fig.1.(a)SEMmicrographofacross-sectionand(b)correspondingXRDpatternofamonolayerCrcoatingdepositedonSisubstratebyDLI-MOCVDaccordingtomode1 (723K;6.7kPa;inhibitorC6H5SH).Thecoatingissingle-phasedandexhibitsatexturedbcc-Crstructure.

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Fig.2. SEMmicrographofthecross-sectionofatwo-phasemultilayercoating grownat673Kand6.7kPaonSisubstrate(accordingtomode2)showingthe9 layerscomposingitanditshighcompactness.

Aboutthetransienteffectsinducedbythesemultilayer deposi-tionconditions,transientchangescanbeexpectedfortemperature and pressure.However thetotal pressureis independently and automaticallyregulatedatafixedvalueanditdoesnotaffectthe microstructureofcoatingswhenitisvariedinanarrowrange.Asa result,thetransientchangesofpressureinducedbythisstepwise deliveryofprecursorareinsignificant.Regardingthesubstrate tem-perature,thetemperatureprofilecalculatedinthis3-dimensionnal reactorshowedacoldfingerattheentranceofthereactoroverafew centimetersoriginatingfromtheinjectionofcoldervaporphase (473K).Thestoppingoftheinjectionoftheprecursorsolutioninthe toluenedecreasesthetotalflowrateofthegasphasebyabout30% (about750sccmcomparedto500sccmwithandwithout precur-sorinjectionrespectively)andconsequentlysubstantiallyreduces thelengthofthistemperaturegradientzoneattheentrance.This transienteffectonthetemperatureisnotharmfulsinceitexpends theisothermalzoneofthereactor.

Bycombining theatomic densitymeasuredbyRBSwiththe thicknessofthecoating(2mm)measuredbySEManditsatomic compositionmeasuredbyEPMA,adensityof7.7±0.6g/cm3 was found.Usuallythedensityofcoatingsislowerthanthatofbulk materials. Despite a significant experimental error,the density measuredisofthesameorderofmagnitudeasthatofbulk bcc-Cr(7.19g/cm3_)._This_confirms_the_high_density_of_DLI-MOCVD_Cr

multilayercoatingsgrownaccordingtomode2.

ChemicalanalysesofpreviousMOCVDandDLI-MOCVDcoatings typicallyrevealedalowcarboncontent(from3to8at.%)[14,22,24]

andaslightcontaminationbyoxygenbecauseofaresidualoxygen inthereactororimpuritiesintheprecursorandinhibitor. Depend-ingonthegrowthconditions,coatingsalsocontainasmallamount ofchlorine(<1at%)orsulfur(<3at%)originatingfromtheinhibitors ofcarbideformation.

In this work, the two types of coating deposited according to modes 1 (monolayer) and 2 (multilayer) using thiophenol as inhibitor of carbon incorporation have similarcompositions whichareingoodagreementwithpreviousstudies[14,22,24].For instance,EPMAanalysisofmultilayercoatingsdepositedat673and 723Kunderlowpressure(6.7kPa)andusingamoleratio thiophe-nol/BEBCequalsto2%intheinjectedsolutionhavethecomposition Cr0.92C0.04O0.03S0.01.

Fig.3.XRDpatternsofatwo-phasecoatingcomprisingthemetastabled-Crphase andthestablebcc-Crphase(topandblackpattern;multilayercoating)andofastable bcc-Crmonolayercoating(bottomandgreypattern)depositedbyDLI-MOCVDon Sisubstrate,respectivelyat673Kand723K(otherconditions:6.7kPa;C6H5SH).

3.2. Evidenceofthemetastablephaseı-Cr

ForallpreviousMOCVDfilmsdepositedat623and673Kwith C6Cl6asinhibitor,thestablebcc-Crphasewastheonlyone

identi-fiedbyXRD[14,18,19].ThesameresultwasfoundbyDLI-MOCVD usingC6Cl6inthetemperaturerange748–793K[22–24].The

coat-ingsgrownat748Kusingthiophenolasinhibitoralsoexhibita single-phase bcc structure [24]. In the present work, thin film growthbyDLI-MOCVDusingthiophenolat673and723Kallowed toobservetheformationoftwocrystallinephases:thestable bcc-Croneandthemetastabled-Crphasewhichhasacubicstructure (space group Pm-3n) witha larger latticeparameter of 4.59Å. TheXRDpatternofapartiallymetastablecoatinggrownat673K accordingtomode2(multilayers)isshowninFig.3andcompared withapurestablebcc-Crmonolayercoatinggrownat723K(mode 1).

For multilayer coatingsdeposited at 673K, both stable and metastablephasesareidentified,whateverthesubstrateused(Si orstainlesssteel).Thestablebcc-Crphaseshowsapolycrystalline structureconsistingofwellcrystallizedgrainswhoseaveragesize islargeenough(>300nm)nottoinduceameasurablewideningof theXRDpeaks.Onthecontrary,theXRDpeaksofthemetastable d-CrphasehavelargeFWHMwhichreflectsthenanometricaverage sizeofthecrystallites,estimatedat17and28nmusingtheScherrer equationfromthepeaks(211)and(200),respectively.

FromtherelativeintensityofexperimentalXRDpeaks(Ihkl)of

Fig.3comparedtothoseofPDFfilesofrandomlyorientedcoatings (I0

hkl),atexturecoefficient(Thkl)forbothphaseswascalculated

accordingtoThkl=(Ihkl/I0hkl)/[n·S(Ihkl/I0hkl)],wherenisthenumber

ofdiffractionpeaksconsideredinagivenangularrange. Contrar-ilytocoatingsgrowninthemonolayermode,multilayercoatings grownat673Kdonotexhibitpreferentialorientationforthe dom-inantbcc-Crphase;T110=0.9comparedto1.8forthemonolayer

coating.Thisisdirectlytheresultofthemultilayerstructurewhich hindersthecolumnargrowth,decreasestheaveragegrainsizeand subsequentlyleadstoahigherdensityofcoatings.Forthis two-phase coating, the metastable phase d-Cr exhibits a noticeable preferentialorientationinthe[211]directionwithanestimated

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Fig.4. InsituXRDofapartiallymetastablechromiummonolayercoatingat dif-ferenttemperaturesunderAratmosphere(depositionat673Kand6.7kPaonSi substrate).Arrowsshowdisappearanceofthemetastabled-Crphase.Thesample washeatedwithatemperaturerampof1Kmin−1_and_it_was_maintained₃₅_min_at

eachtemperaturetorecordthepattern.

texturecoefficientof1.8despitethelowintensityofthediffraction peaks.

TheXRDpeaksofthemetastablephasearealwayslessintense thanthoseofthestablephase,indicatingsmallamounts.Fromthe ratiooftherelativeXRDintensityofthemostintensepeaksofd-Cr (211)andbcc-Cr(110),andsoneglectingthelowtextureofthe metastablephase, acoarseproportionofd-Crwasestimatedat about6%.Thismethodsuffersfromtwoapproximations;(i)the lowtextureofd-Croverestimatestheintensitythatshouldbe con-sideredford-Cr(211)and(ii)thecrude intensityofbcc-Cr(110) isalsooverestimatedbecauseofoverlapwiththed-Cr(210)peak. Interestingly,bothapproximationscompensatewhatmakessense tothemethod.

InsituXRDanalysesallowedustheverificationofthethermal stability,inargonatmosphere,ofthemetastablephaseembedded inthestablebccmatrix.ThearrowsonXRDpatternsofFig.4show thecompletetransformationofthemetastabled-Crphaseintothe stablebcc-Cronebeyond723K,ingoodagreementwithliterature data[25–27].Tracesofchromiumoxideappearabove823K,mainly duetotheresidualoxygenpartial pressurein theXRDanalysis chamber.Onreturningtoroomtemperature,theXRDpatternis thesameasat873Kconfirmingthatthed-Cr→a−Cr transforma-tionisirreversible.FromthisseriesofXRDpatternsrecordedinsitu onadiffractometerdifferentfromthatofFig.3,themean crystal-litesizeestimatedwiththed-Cr(211)peakis21,32and30nmat thetemperatures623,673and723Krespectively.Thesevaluesare consistentwiththatmeasuredatroomtemperaturewithdataof

Fig.3.Nosignificantvariationoftheaveragesizeofd-Cr crystal-liteswasfoundforthisheattreatment.Above723K,themetastable phasedisappears.

Noevidenceforthemetastabled-CrphasewasfoundbyTEM analysis,probablybecauseoftheverylowamountofthisphase comparedtothestableone.Anotherhypothesis isthatitcould undergoaphasetransformationundertheTEM200kVelectronic beam. Electron-beam induced phase transformation is a well-known phenomenon as reported in several papers relative to metallicalloys[28],minerals[29]oroxides[30].

Fig. 5 illustrates a TEManalysis of a two-phase monolayer coatingcharacterizedbyacolumnarmorphology.APrecisionIon PolishingSystem(PIPS)wasusedtopreparethesample,resulting inathinsliceofthecoatingapproximatelyparalleltothesurface ofthefilm.ThereforeFig.5ashowsacross-sectionofonecolumn (andapartofanotherone)perpendiculartoitslongitudinalaxis. TheinsertinFig.5bpresentsthecorrespondingelectron

diffrac-tionpattern attributedtothestablebccphase with[011]zone axis.Anexperimentallatticeparameterof2.92Åwasdeducedfrom electrondiffractionpatternsofseveralcrystallitesingood agree-ment, within2%,withthe literaturevalueof bcc-Cr(2.88Å).A high-resolutionpictureinthesameareashowninFig.5crevealsa veryhomogeneouspacking.ItsFouriertransformintheinsert(d) isinagreementwiththeexperimentalpatternin(b).Moreover, columnsobserved by TEMappear polycrystallinein agreement withthefactthatthetextureisnotveryimportantevenifitexists formonolayercoatings(Fig.1b).Thecross-sectionconfirmsthese columnshaveanaveragediameterofabout1mm,asfoundinSEM observations(Fig.1a).

A two-phase multilayer coating comprising the metastable componentwasanalyzedbySEM,coupledtoanEBSDdetector. Itisinfactacross-sectionofthismultilayercoatingpreparedwith across-sectionpolisher(JEOLIB-19510CP)whichwasstudied.An EBSDbandcontrastexampleispresentedonFig.6a.Itissimilarto classicalsecondaryelectronsSEMimages,asshowninFig.2,except thatthecontrastmakesiteasiertoseethegrains.EBSDphase map-pingis showninFig.6b.Thestablebcc-Crandmetastabled-Cr phasesareidentifiedwithtwodistinctcolors,respectivelyblueand red.

EBSDstatisticalanalysiswasperformedonapproximately1000 grains.IdentificationofbothmetastableandstableCrphaseswas possible.Themetastabled-Crphaseproportionisaround3%.This comparesquitewellwiththe6%determinedbyXRDassumingthat bothphaseswerenottextured.Themetastablephaseisessentially foundatinterfacesofthemultilayerandatgrainboundariesofthe dominantbccphasewherethereisthehighestdensityofstructural defectsandinlowEBSDsignalareas.Itwascomplicatedto dis-cernthetwophasesbecausetheyhavethesamecompositionand bothexhibitcubicstructures(bccforthestablephaseandprimitive cubicforthemetastableone).Inthistwo-phasemultilayercoating, theaveragegrainsizeofbcc-Crisapproximately300nmwhileitis onlyafewnanometersford-Cr.Inapreviouswork,Itwasreported thattheparticlesofthismetastabled-Crphasedidnotexhibita definitecrystalshapeandtheiraveragesizewaslowerthan20nm

[26].

NospecificgrainorientationwasobservedbyEBSDanalysisfor thestablebccphase,asitcanbeseenontheinversepolefigurein

Fig.6c.Thisconfirmsthatnopreferentialorientationofthegrowth occursusingthismultilayerCVDmode.Thisresultisinverygood agreementwithXRDanalysiswhichshowedtheabsenceoftexture forthestablephasewithatexturecoefficientof0.9forthe[110] direction(Fig.3).

XPSanalyseswereperformedtostudytheenvironmentofatoms andanalyzethenatureofimpuritiesinthesecoatings.Asurvey scanafterthesurfacecleaningbyAr+_sputtering,_reported_as

sup-plementaryinformation,revealsthepresenceofonlyCr,O,Cand S(Fig.S1).Itisnotpossibletodistinguisha-Crfromd-Cr.Fig.7

presentsXPSregionspectraofCr2p,O1s,C1sandS2plevelsfrom atwo-phasemultilayerCrcoatingdepositedonaSisubstrate.Three differentsputteringtimesaredisplayedforeachelectroniclevel: 0s(as-deposited),120sand2012s.As-depositedsampleexhibits asurfacecontaminationcontainingchromiumoxides(Cr2p3/2at

576.6eVandO1sat530.8eV),adventitiouscarbon(C1sat284.8 and288.3eV)andsulfateanions(S2p3/2at168.7eV).

Forthelongestcleaningtimebysputtering,thiscontaminated surfaceissufficientlycleanedanddataaremorerepresentativeof thecoatingcomposition.ThemaincontributionofCr2p3/2isshifted

to574.0eVwhichischaracteristicofmetalCr,evenifanoverlap existswithasmallcontributionduetoCr Cbonds(Fig.7a).Traces ofoxygenleadtoaresponseofCr Obondsasashoulderat576.6eV forCr2p3/2andacontributiontoO1sat530.8eV(Fig.7b).This

oxy-genwasattributedtoaslightoxidationofthecoatingsinagreement withEPMAdata(3at%).Fig.7cshowscarbonincorporatedintwo

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Fig.5.(a)TEMobservationofatwo-phasemonolayerCrcoating;(b)correspondingselectedareaelectrondiffractionpattern([011]zoneaxis);(c)Highresolutionmicrograph; (d)correspondingFouriertransformofthehigh-resolutionview.

Fig.6. EBSDanalysisofacross-sectionofatwo-phasemultilayercoatinggrownat673Kand6.7kPaonSisubstrate:(a)bandcontrast;(b)phasemapping;(c)inversepole figure(zaxis).

forms:asfreecarbon(C1sat284.6eV,dominantcontribution)and ascarbide(C1sat282.8eV).Bycombiningtherelativeintensityof freeCandcarbidecomponentswiththetotalCcontentdetermined byEPMA(4at%),theamountofCinthecarbideformisestimated atabout0.8at%.ThisissignificantlyhigherthanCsolubilityinCr whichwasreportedtobe<0.1at%atabout750K[31].

Aftersurfacecleaning,theS2plevel,althoughnotveryintense, givesevidenceforauniqueenvironmentintheformofS Crbonds withS2p3/2shiftedat161.6eV,withashoulderat162.8eVforS

2p1/2(Fig.7d).ThisischaracteristicofmetalsulfideorofS

solubi-lizedinthemetalstructure.Inthissample,totalScontentanalyzed byEPMAwasonly0.5at%whichishigherthanthesolubilityof sulfurinchromiumat673K[32].Consequentlyasulfide

contam-inationlikelyexistsbuttheamountistoolowtobeanalyzedby XRDorTEM.

3.3. Originofthemetastablephaseı-Cr

Themetastablephased-Crwasclearlyidentifyinsmallamount byXRDandEBSDinDLI-MOCVDCrmetalcoatingsdepositedboth inmode1(monolayer)andmode2(multilayer).Itisbetter visi-bleinmultilayercoatingsthaninmonolayerbecausethecolumnar growthand textured structure in thelattermake it difficult to observe,sincetherelativeintensityofd-CrXRDpeaksiscrushed bythatofthepreferentialorientationofa-Cr.Severalfactorsseem toinfluencetheformationofthismetastablephase.

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Fig.7. XPSanalysesofatwo-phasemultilayerCrcoatingcomprisingthemetastabled-Crandthestablea-Crphasesdepositedat673K.Spectralregionsof(a)Cr2p,(b)O 1s,(c)C1sand(d)S2parereportedfordifferentAr+_sputtering_times₍₁_keV):₀_s_{(as-deposited;}_black_spectrum,_bottom),₁₂₀_s_(blue,_middle)_and₂₁₂₀_s_(red,_top)._An_offset

alongtheYaxishasbeenappliedforclarityandadeconvolutionofthemostrepresentativespectrumofthecoatingisplotted.(Forinterpretationofthereferencestocolour inthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)

It wasnever reported in Crcoatingsgrown by MOCVD and DLI-MOCVDusing chlorinatedinhibitors[14,17–19,22–24] even fordepositiontemperatureaslow as573K.Whenusing sulfur-containinginhibitorsitwasnotmentionedbyMOCVD[21]norby DLI-MOCVD[24](Table2).Ithasbeenobservedinthisworkby DLI-MOCVDusingC6H5SHasinhibitorofcarbonincorporationonlyfor

depositiontemperature≤723K.Thisisingoodagreementwiththe factthatbeyondthiscriticaltemperatureastructuralphase trans-formationintobcc-Croccurs,asreportedby[25–27]andconfirmed byinsituXRDanalysisinthisstudy(Fig.4).

Whenthedepositioniscarriedoutat723K,theobservation ofthisphasedependsonthepositionofthesamplesina quasi-isothermalreactor,whichmeansitdependsonthelocaldeposition conditions,i.e. mainlyonthe substratetemperature.Numerical modeling(notreportedhere)showedforourDLI-MOCVD reac-torthatwhenthetemperatureofthefurnaceissetto723K,the substratetemperaturewasapproximately50Klower,i.e.around 673K.Thistemperaturedecreasecomesfromofthepresenceofa coldfingerattheentranceofthereactoroverseveralcentimeters intheaxialdirection[33].Thisisduetothelowertemperatureof thereactivegasphase(approximately473K)comingfromtheflash vaporizationchamberwhichentersthereactor.Consequently,the formationofthemetastablephaseisthenpossiblebecausethe sub-stratetemperatureislowerthanthecriticalvaluebeyondwhich thestructuraltransformationoccurs.Forthesamefurnace

tem-perature(723K),butusinganothergeometryandsizeoftheCVD reactordifferenttransportconditionshavetobeusedleadingto differenttemperaturefields.Consequently,thetemperature dif-ferencebetweenthesubstrateand thefurnaceset-pointwillbe differentthaninthisworkandthemetastablephasemaynotbe observedifthereactorismoreisothermal.Thisisconsistentwitha sharpstructuralphasetransformationatthiscriticaltemperature of723K.So,forthedepositionofmetallicCrcoatingscontaining d-Crasaminorphase,DLI-MOCVDisasuitableprocess.The sub-stratetemperaturemustbelowerthan723KandC6H5SHisthe

bestinhibitor.

Structuralphasetransformationinmetalsinducedby impuri-tiesisacommonphenomenon[34].Moreover,magneticordering of chromium is affectedby impurities (doping) which induces changesinphasetransition[35].Thiophenollikelypromotesthe formationofthemetastablephase.Sulfurimpuritiesare incorpo-ratedinthecoating(<1at.%)andtheycouldinducethestructural phasetransformationwithinCrcoatings.Thiophenolactionwas foundefficientstartingfromamoleratiothiophenol/BEBCof2% (thetestedvalueswereintherange1–10%).Foramoleratioof only1%,noinhibitoryactionwasobservedsincetheformationof chromiumcarbidesoccurredinsteadofmetallicchromium.

TherelativelylargeFWHMofXRDpeaksofd-Crphasetogether withtheEBSDanalysisgivesevidencethatthismetastablephase ispresentintheformofnanocrystallites.Thenanometricsizeof

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d-Crparticlesmaybeanothercauseofitsstability.Indeedsucha nanometriceffectisknownforinstancetostabilizethemetastable anataseform of titania relative torutile although this last one isthermodynamicallythemoststablephase.Thisisbecausethe surfaceenergyofanataseissignificantlylowerleadingtostable anatasenanoparticlesbelowacriticalsizeofabout15nm[36].Our observationofd-Cronlyintheformofnanoparticlesisconsistent with[26]whereauthorsalsoreportedd-Crasnanoparticles,which couldbeexplainedbyalowersurfaceenergythanthatofbcc-Cr.

Asmentionedabovethed-Crphaseismoreeasilyseenin multi-layercoatingsthaninthemonolayercoatings.Furthermore,EBSD analyseshaveshownthatitislocatedbothattheinterfacesand grainboundaries(Fig.6).Fromfundamentalsof nucleationand crystalgrowthitcanbeassumedthatthemetastablephase depo-sitioniscontrolledbyaheterogeneousnucleationstepratherthan asteadystategrowthregimebecausethesurfaceseemstoplay animportantroleanditscrystallitesizestayslimitedatthe nano-metricscale.Thishypothesissuggeststhatitsproportionwould increasewiththenumberofgrainboundariesandinterfacesas obtainedinthemultilayercoatings.Sobyincreasingthe nanos-tructurationwhichmeansbyincreasingthenumberofindividual layersinthecoating,thenumberofbothgrainboundaries(dueto loweraveragecrystallitesize)andinterfaceswillincrease,which couldfavorthegrowthofd-Cr.Thisideahasnotyetbeenverified. FromtheXRDpatternofFig.3,theproximityofthemostintense peaksofeachphase suggeststhatanepitaxialrelationshipmay existwhichwouldfavorthenucleationofd-Cr(211)planeon a-Cr(110)andwouldexplainthepreferentialorientationofd-Crin the[211]direction.

3.4. Unusualproperties

These DLI-MOCVD Cr coatings are promising candidates as protectivemetallurgicalcoatings.Theyaredepositedatlow tem-perature(T<723K)witharelativelyhighgrowthrate(∼5mm/h). The multilayer coatings exhibit a density as high as that of bulkmetallicCr(7.19g/cm3_)._For_applications_in_harsh

environ-ments,iftheoperatingtemperatureincidentallyexceeds723K,the metastablephasewillbeirreversiblytransformedintothestable bccphase(Fig.4).Accordingtotheircrystallographicstructures,the metastabled-Crphaseisslightlylessdensethana-Cr(7.16g/cm3_).

Thismeansthatatthetimeofthephasetransformationtheoverall volumecontractionwillbeonly0.4%.Furthermoretheproportion ofd-Crdoesnotexceed6%inmultilayercoatingsasdeducedfrom XRDandEBSDanalyses.Thisindicatesthatitcanactasatracer likely withoutloss of thebasicproperties of thecoatings.This isparticularlyusefulinnon-destructivetestingofmanufactured structuralcomponentsprotectedwithsuchacoating.

Indeed carbon-steels and alloys undergo specific heat treatments to optimize their mechanical properties (ductility, hardness...) and theiroperating temperaturemust not exceed the temperatureof the last treatmentundergone as hardening ortemperingannealing.Beyondthecriticaltemperatureasteel structurecannotcarrytheserviceloadforwhichitwasdesigned. Thephasetransformationofd-Croccursinthetemperaturerange oftheheattreatmentsofmanysteelsandmetallicalloys,asfor instanceZircaloy-4 usedin nuclearindustrywhich willloseits metallurgicalstateascladdingmaterialabove753K[37].IfaCr coatingisdepositedbyDLI-MOCVDonthisalloy,the disappear-anceofthemetastablephased-Crwillrevealincidentalconditions inservicegreaterthan723Kwhichwillalsoaffecttheproperties of thezirconiumalloy.Obviously, theexposuretime alsoplays animportantroleinthisd-Cr/a-Crtransformation,butakinetic studyofthistransformationwasnottheprimarypurposeofthis paperanditwillbeinvestigatedshortly.

Preliminary results of mechanical properties were obtained usingananoindenter(Nanoscratchtester,CSMinstrument)for monolayer and multilayercoatings deposited onsteel at 673K usingC6H5SHasinhibitor.ThedataarereportedinTableS1

(Sup-plementary material) and are compared to previous values for similarcoatings.The nanohardness ofa columnar 3.5mmthick monolayercoatingis9.7GPa.Thisisconsistentwiththe13.0GPa found for columnarmonolayer coatingsdeposited at thesame temperaturewithC6Cl6asinhibitor[18].Moreinterestingfor

tech-nologicalapplicationisthehighernanohardnessof16.9GPafound for a 5.5mm thickmultilayer coatingthat exhibitsa high den-sity(depositedaccordingmode2).WhenCrcoatingsgrownby MOCVDandDLI-MOCVDusingC6Cl6 asinhibitoraresufficiently

dense,theyalsoexhibitacomparablehighnanohardnessof19.0

[18]and17.0GPa[22],respectively.Whatevertheinhibitorused (C6Cl6 orC6H5SH)densecoatingshaveanelasticmodulusinthe

range270–310GPawhichisclosetothe285GPaofbulkCr. Further-moretheyallexhibitacompressiveresidualstresscloseto0.6GPa (determinedfromthecurvaturechangeofsamples).

Wewillnolongercommentonthesepreliminaryproperties,but therearetwostrengthsthatmustberetained:(i)DLI-MOCVDCr metalcoatingsaremuchharderthanthosedepositedbyother tech-niques,e.g.electrodeposition[1–3]orarcevaporation[8],and(ii) thereisnosignificantdifferenceforMOCVDandDLI-MOCVD coat-ingsdependingonthenatureoftheinhibitorused,whichmeans thatthepresenceofthemetastablephased-Crdoesnot signifi-cantlyaffectmechanicalproperties.Theveryhighhardnessofthe sameorderofmagnitudeasthatofchromiumcarbidesisprobably duetocarbonsupersaturationinCrasrevealedbyXPSanalyses (Fig.7).Thefactthatthemetastablephasewouldnotaffectthe mechanicalpropertiesofthetwo-phasecoatingswouldbedueto thecubicstructureofeachcomponent,theirverysimilardensity andthesmallamountofd-Cr.

4. Conclusions

Alow amount(<6%)ofthecubicmetastabled-Crphase was foundmixedwiththestablebcc-CrphaseinDLI-MOCVDcoatings. Theformationofthismetastablephaseresultsfromboththelow temperatureofdeposition(<723K)andtheuseofthiophenolas inhibitor ofcarbide formation.It wasnot reportedunderother CVDconditions.Densecoatingsweredepositedbyimplementing amultilayergrowthmodeinordertoavoidacolumnarandporous morphology.Suchcoatingsexhibitahighnanohardnessofabout 17GPa.

Accordingtotheliterature,thismetastablephaseisirreversibly transformedintothestablebcconefrom723Kininertatmosphere. Thisstructuraltransformationshouldnotaffectthepropertiesof thecoatingduetothesimilarityoftheircrystallographicstructures (bothcubic)andtheirveryclosedensity(avolumecontractionof only0.4%atthetimeofstructuraltransformation).Thepresence ofthismetastablephaseconstitutesasignatureoftheDLI-MOCVD process.Itcanbeusedasatracerforcoatingsoperatinginhigh temperatureenvironmentwithoutlossofthebasicpropertiesof thecoatings.Thisisparticularlyusefulinnon-destructivetesting ofmanufacturedstructuralcomponents.

Acknowledgements

The authors thank Dr. Arnaud Proietti and Marie-Christine LafontfortheirhelpinEBSDandTEManalysesrespectively,and R.Laloofornanohardnessmeasurement.RBSanalysesweremade byH.GuéganinARCANE-CENBG,Gradignan,France.Thisworkwas partiallysupportedbyCNRS,CEA,INPTandtheCentreofExcellence ofMultifunctionalArchitecturedMaterials(CEMAM).

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AppendixA. Supplementarydata

Supplementarydataassociatedwiththisarticlecanbefound,in theonlineversion,atdoi:10.1016/j.apsusc.2017.05.253.

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